1. Field of the Invention
An aspect of the present invention relates to an optical scanning apparatus for use in image formation in an image forming apparatus such as a laser printer, and more particularly, to a collimating lens, which compensates for a change in temperature during use, and thus, causes no variation of focal distance, and an optical scanning apparatus using the same.
2. Description of the Related Art
Generally, an image forming apparatus such as a laser printer or a digital copier uses an optical scanning apparatus to form an image on a photosensitive medium. Accordingly, a light is emitted from a light source in accordance with image signals, and modulated as it passes through a collimating lens and a cylindrical lens to be periodically deflected on the light deflector such as a rotatable polygon mirror. An image is recorded as the deflective light from the light deflector is focused on the surface of the photosensitive medium through an fθ lens in the form of a spot.
With the ongoing development in the field of the image forming apparatus, such as printers, an optical scanning apparatus having a smaller F-number or spot size has been in increasing demand. Accordingly, an optical system having a smaller F-number has been developed. However, an optical system with the small F-number is accompanied by problems, such as reduction in depth of focus, which subsequently causes spot size on the surface of the photosensitive medium to vary with the changes in temperature.
Some suggestions to solve the above problems have been made, including: properly combining materials for the collimating lens and the lens barrel, properly distributing material and power for the plurality of lenses of the collimating lens and employing diffraction elements.
However, adjusting the material for the lens and the lens barrel has resulted in mechanical limitations, and the use of diffraction elements has not been provided with practical designs and fabricating methods thus far.
A method of properly distributing material and power for the collimating lenses is disclosed in Japanese Patent Publication No. 2002-6211, which comprises a temperature-compensating lens and an optical apparatus using the same.
FIG. 1 illustrates a light source unit of an optical scanning apparatus using the temperature compensating lens of JP 2002-6211.
Referring to FIG. 1, the light source unit includes a light source 10, a collimator lens 20, a diaphragm 30, and a housing 40.
The light source 10 emits a ray of light with the reception of electricity thereto, of which a laser diode is a typical example thereof.
The collimator lens 20 includes at least one piece of positive lens and at least one piece of negative lens, with more than two pieces of lenses being made of different materials. In the order from the end of the collimator lens 20 where the light is emitted, with an index of refraction of ith lens material being ni, variation of ni with respect to temperature rise dt being dni/dt(1/° C.), and power of the ith lens being φi(1/mm), the inequality 1 is satisfied.
                                          ∑                          i              =              1                        m                    ⁢                                          ⁢                                                    Φ                i                                            (                                                      n                    1                                    -                  1                                )                                      ⁢                                          ⅆ                                  n                  i                                                            ⅆ                t                                                    ≤        0                            Inequality        ⁢                                  ⁢        1            
With the collimating lens 20 satisfying the inequality 1, a focal point is easily compensated because expansion of the positive and the negative lenses offset each other.
However, according to the above-described patent publication, the collimating lens 20 requires at least two glass lenses, preferably four glass lenses, which have different indices of diffraction as an essential feature. Accordingly, since there are a number of parts required, the weight of the apparatus also becomes heavier. As a result, the cost of manufacturing as well as the assembling process increases.
Consequently, there have been increasing demands for a collimating lens in which a focal point is compensated in response to the rise of ambient temperature.